Some experts are unsure of the ability of protein-based tests to predict the return of thyroid carcinomas, which is why a team of scientists recently monitored the recurrence of papillary thyroid cancer (PTC) or follicular thyroid cancer (FTC) among patients given serum thyroglobulin (Tg) assays.

The results, which appeared in the Journal of Clinical Endocrinology and Metabolism, indicated that Tg tests predicted the reappearance of PTC and FTC with significant accuracy.

PTC and FTC are the two most common types of thyroid cancer. Both have relatively good survival rates.

The American Cancer Society (ACS) estimates that five years after diagnosis, 100 percent of people with Stage I or II PTC will still be alive. The same goes for patients with FTC of either stage.

Even the most advanced cases of these two diseases have fairly good outcomes. For people with Stage IV PTC or FTC, the five-year survival odds are 51 and 50 percent, respectively.

Compare those figures to the estimated outcome for patients with anaplastic thyroid cancer (ATC), the most virulent carcinoma of the gland. The ACS states that any case of ATC - a disease automatically considered Stage IV - has a 7 percent survival rate over five years.

In the new study, more than a dozen nuclear medicine specialists examined the effectiveness of Tg blood tests at predicting the recurrence of the relatively mild PTC and FTC. To do so, the team monitored the health of more than 700 people diagnosed with either disease.

All cases of thyroid cancer were localized, meaning that regardless of their stages, none had yet spread beyond the affected gland. Doctors screened participants for metastasis by using radioactive iodine uptake tests.

After receiving total thyroidectomies, all patients were given a clean bill of health. The team then put each individual on either the synthetic hormone levothyroxine (LT4) or thyroid-stimulating hormone (TSH) therapy.

After three months, PTC patients were given a Tg test. The assay looked at levels of thyroglobulin, a protein whose excess presence in the blood can indicate a risk of thyroid cancer.

The same exam was given to those treated for FTC, though after an interval of between nine and 12 months. The team then waited for several years before seeing the patients again.

Ultimately, researchers found that 32 participants got cancer a second time.

By analyzing data collected in earlier exams, the team determined that the Tg tests accurately predicted the recurrence of thyroid cancer, regardless of the form of hormone therapy an individual had been given. Tg tests for patients taking LT4 ended up having a predictive specificity of 84 percent, and those for people on TSH had a predictive specificity of 99 percent.

Scientists concluded that the results of a post-thyroidectomy Tg test are a reasonable indicator of the likelihood of a recurrence of PTC or FTC.

How likely is thyroid cancer to recur? The Columbia University Department of Surgery (CUDS) says that it depends on a tumor's stage.

A Stage I thyroid carcinoma has a 2.8 percent chance of reappearing elsewhere in the body, the CUDS states, while a Stage IV tumor is almost certain to metastasize outside the thyroid.